1. Field of the Invention
The present invention relates to a free space optical communication link, and in particular, the invention relates to techniques to spread error sources over time intervals and provide redundant channels to reduce the effects of fading.
2. Description of Related Art
Known optical communication systems rely on optical fibers between transmitter and receiver. However, to establish a system network require obtaining right of ways and installation of fiber, a time consuming and expensive process.
Free space optical communication systems are fundamentally different than fiber optic systems in that fiber is typically used to transport data between node points. Distances are more limited with free space due to atmospheric attenuation or man-made or natural obstacles. In particular, in free space, the media is air and subject to atmospheric disturbances such as fog, rain and resulting fades. Please refer to our earlier United States patent application with Robert Miller, U.S. Ser. No. 09/640,576 filed Aug. 17, 2000, entitled xe2x80x9cFree Space Optical Communication Link with Diversity,xe2x80x9d to be deemed incorporated by reference as to its entire contents
Koh and Davidson (xe2x80x9cInterleaved Concatenated Coding For The Turbulent Atmospheric Direct Detection Optical Communication Channelxe2x80x9d, IEEE Transactions On Communications, Vol. 37, No. 6, June 1989, pages 648-651) discuss how the direct detection atmospheric optical communication channel is characterized by strong fading of the received laser light intensity caused by random variations in the index of refraction encountered by laser light variations as it propagates through the channel.
In addition, the Jet Propulsion Laboratory of the California Institute of Technology published in November of 1998 a Technical Support Package on Multiple-Beam Transmission For Optical Communication in November 1998 as NASA Tech Brief, Vol. 22, No. 11 from a JPL New Technology Report NPO-20384. This NASA Tech Brief describes how superposition of mutually incoherent beams would reduce deleterious effects of atmospheric turbulence.
It is an object to the present invention to provide a free space wavelength duplexed optical communication link that is reduces the effects of fading.
This and other objects are achieved in a free space wavelength duplexed system that includes first and second terminals. The first terminal includes an optical transmitter and an optical receiver. The optical transmitter provides multiplexing. The optical receiver has an optical system that functions similarly to a telescope for collecting light emitted by the corresponding transmitter, an optical to electrical converter and an optical amplifier coupled between the optical system and the optical to electrical converter.
In an alternative embodiment, a method includes the steps of receiving a received optical signal through a telescope, diverting the received optical signal in an optical splitter into an optical amplifier, and transmitting a transmit optical signal through the optical splitter to the telescope.
In another alternative embodiment, a method includes the steps of receiving plural received optical signals through a telescope, diverting the plural received optical signals in an optical splitter into an optical amplifier, separating the plural amplified optical signals by wavelength, and transmitting plural transmit optical signals at distinct wavelengths through the optical splitter to the telescope.
The receiver includes diversity reception means to optimally combine the received signals.